Porous gas fired radiant burners have been in use for many years. The burners are an inexpensive source of radiant energy when compared to comparable units utilizing electric resistance heating. These burners are used in numerous industrial applications such as paint and paper drying. Additional applications include, for example, the heating of breezeways in colder climates.
The burner consists of a porous plate acting as one side of a box. The other five sides act as a plenum chamber to divert a mixture of gaseous fuel and air through the porous plate. The fuel-air mixture is ignited at the surface of the porous plate and combustion proceeds at the plate surface. The pore structure of the plate is fine enough to prevent flashback of the burning fuel-air mixture into the plenum chamber. FIG. 8 illustrates such a burner.
As surface combustion proceeds, the surface temperature of the plate rises. The ultimate temperature the porous plate attains depends on the plate thickness, its porosity and the amount of fuel-air mixture flowing through it. The amount of radiant heat produced by such a plate is proportional to its surface temperature. Unfortunately, many of the materials used to fabricate these porous burner surfaces will not withstand the higher operating temperature imposed by running the burner at high surface temperatures to achieve higher radiant output.
Assuming complete combustion (and ignoring conductive heat transfer), the available heat produced for a given quantity of fuel-air is the sum of the radiant and convective energy fractions. Placing of a refractory screen or grid at a short distance from the porous plate burner surface has been a common way to boost the radiant output of the burner by converting some of the convective energy into radiant energy. This phenomenon is termed a "reverberatory" effect; thus, such screens are reverberatory screens. This allows the burner surface temperature to be reduced for a given radiant output, greatly increasing the useful life of the porous burner panel. Further, the presence of such a reverberatory screen permits the radiant burner to be operated more efficiently. Specifically, some of the convective heat energy which might otherwise be lost (hot air rises) is converted to radiant energy, which is more easily directed to where it is needed.
The reverberatory screen is prone to the deleterious effects of high temperatures much in the same way as is the porous burner panel. Screens are commonly made from refractory metals such as Nichrome.RTM. and Inconel.RTM.. In some cases the screens are treated with oxidation protection coatings such as pack aluminization to allow them to operate at higher temperatures for extended time periods.
Because some components of a radiant burner system are designed to operate at different temperatures than other parts and may be constructed from different materials, thermally-induced mechanical stresses are usually generated during operation of the burner. Depending upon how the various components and sub-assemblies of the burner are attached to one another, these stresses may produce physical distortion of the burner parts. Particularly vulnerable is the reverberatory screen. Attachment plans have been developed over the years in an effort to minimize the degree of stress and distortion. Some of these schemes are rather complex, requiring numerous pieces which adds to cost. Eventually, however, metal reverberatory screens become thermally distorted (e.g., from creep deformation) and have to be replaced. Often, these radiant burners are designed and operated such that screens are sacrificed to preserve the porous burner panel, which is considerably more difficult to replace.
In addition to the stress-induced distortion problem, there are other limitations of metallic burner hot-stage components. Specifically, such components are oxidation-prone. Oxidized metal tends to be brittle. Also, the oxidized layer may not be adherent to the underlying metal. The higher the temperature, the more rapidly the oxidation reactions proceed. Thus, even though increasing the operating temperature would increase the thermodynamic efficiency of the burner, the chemical and mechanical limitations of metals impose a cap on the practical operating temperature of the radiant burner.